1. Field of the Invention
The present invention relates to an apparatus for and a method of recording an image on a photosensitive medium by scanning the photosensitive medium with a light beam.
2. Description of the Related Art
There has heretofore been known an image recording apparatus for recording an image on a photosensitive medium such as a film by scanning the photosensitive medium with a light beam, as disclosed in Japanese laid-open patent publication No. 63-191473, for example.
The above publication refers to the following problem: When a halftone dot recorded on a film by a light beam by way of exposure is developed, it usually becomes slightly greater than the recorded halftone dot due to the Gaussian distribution of a light spot on the film. This phenomenon tends to be more noticeable when the developing liquid used to develop halftone dots is deteriorated. It has been described in the publication that since the extent by which the halftone dot becomes larger remains substantially constant irrespective of the size of the halftone dot, if the halftone dot is of a small size whose halftone dot % (hereinafter referred to as halftone %) is 10% or less, the effect that the larger halftone dot has on an image representation is greater than if the halftone dot is of a size for a medium gradation.
To solve the above problem, the publication discloses a light beam intensity correcting circuit as shown in FIG. 18 of the accompanying drawings. As shown in FIG. 18, the light beam intensity correcting circuit has a light beam intensity correcting means 8 comprising a memory (memory table) 2, a D/A converter 4, and a buffer amplifier 6. A multigradation digital image signal a, e.g., a digital image signal a in 256 gradations (raging from 0 to 100 halftone %), is supplied to a comparator 10 and the memory 2 of the light beam intensity correcting means 8.
The comparator 10 compares a threshold signal, which is of a value in the range from 0 to 256, from a threshold memory 12 with the digital image signal a, and outputs a binary signal c, which is of a high level or a low level, indicative of the compared result. When the binary signal c is of a high level, a switch 14 has its common contact shifted to connect the light beam intensity correcting means 8 to an optical modulator 16, as shown.
The optical modulator 16 modulates the intensity of a laser beam depending on an output signal f from the light beam intensity correcting means 8. The intensity-modulated laser beam outputted from the optical modulator 16 is applied to record an image on a film 18.
The memory 2 stores an amount-of-light control value for keeping the amount of light of the laser beam at a constant level when the halftone % is in a shadow and medium range, and an amount-of-light control value for progressively reducing the amount of light of the laser beam when the halftone % is in a highlight range, i.e., 10% or less.
With the conventional light beam intensity correcting circuit shown in FIG. 18, the extent by which a halftone dot becomes greater in the highlight range is reduced depending on the degree of correction of the intensity of the laser beam, resulting in a halftone dot smaller than usual.
The above publication reveals that since the intensity of the laser beam is controlled to reduce the amount of light of the laser beam in the highlight range, the extent by which a halftone dot becomes greater in the highlight range is eliminated, resulting in an increase in the accuracy of the halftone dot representation.
However, the disclosed conventional image recording apparatus is disadvantageous in that since the intensity of the laser beam is corrected in a unit of halftone %, i.e., in a unit of the entire halftone dot, depending on the gradation of the input image signal, the intensity of the laser beam is corrected identically irrespective of the shape of the halftone dot.
Recently, there have been developed various CPT (Computer To Plate) exposure apparatus for performing a platemaking process by directly exposing a photosensitive printing plate to an image. For example, Japanese laid-open patent publication No. 2000-35673 discloses an inner-drum plate setter and a lithographic printing plate. When such a photographic printing plate is exposed to a halftone dot image by a laser beam, a new problem arises if the intensity of the laser beam is controlled to a small degree in a range of small halftone %.
The photographic printing plate comprises a support body supporting thereon a photosensitive layer where an area irradiated by a laser beam remains as a halftone dot image. As shown in FIG. 19 of the accompanying drawings, the photographic printing plate is basically produced from a photosensitive medium 32 which has a photosensitive layer 21 of a photopolymer disposed on a support body 20 of a metal base such as of aluminum or the like, and a transparent overcoat layer 22 disposed on the photosensitive layer 21 for blocking oxygen.
When light is applied to an area 23 of the photosensitive medium 32, the area 23 is hardened. Thereafter, the photosensitive medium 32 is heated to accelerate the hardening of the area 23. After the photosensitive medium 32 is heated, it is developed in an alkaline developing liquid, and the area of the photosensitive medium 32 which is not irradiated by the light is scraped off by a brush or the like, producing a printing plate 25 where the irradiated area 23 serves as an image area 24.
As a result of an experiment conducted on the photosensitive medium 32, it has been found that if the image area 24 is not hard enough and is present as an isolated image area or a small image area, it tends to fall off while the photosensitive medium 32 is being processed in the developing process.
It has also been found that in an image recording apparatus for scanning the photosensitive medium 32 directly with a light beam to form a halftone dot image according to an area modulation (area gradation) process, in order to sufficiently produce an image with small halftone %, i.e., a highlight image, and keep the highlight image with small halftone % sufficiently resistant to plate wear, it is necessary to expose the photosensitive medium 32 to an increased amount of recording light for the highlight image, as indicated by a qualitative characteristic curve 26 in FIG. 20 of the accompanying drawings, unlike the conventional arrangement shown in FIG. 18.
However, when the amount of recording light increases, halftone dots in the shadow range are clustered to reduce the number of resolution points of halftone %, resulting in such a problem that a black thin line within a highlight area and a white thin line within a shadow area have different thicknesses.
As indicated by a qualitative characteristic curve 27 in FIG. 21 of the accompanying drawings, it has also been found that when the amount of recording light increases, the amount of flaring light produced around an image area increases to create a fog in a non-image area, resulting an image irregularity.